The Raptor Center in St. Paul is overloaded with convalescing bald eagles, and more are likely on the way. Officials at the University of Minnesota center say the rising toll on eagles is a troubling and expensive mystery.

Last year the Raptor Center took in 114 eagles. This year, the center's veterinarians have already treated a record 145 birds — almost 50 more than at this time last year — and the busiest hospitalization period is just beginning, as eagles become poisoned by ingesting lead ammunition from deer organs left in the field by hunters.

All 12 eagle cages at the center are full, and 13 more eagles are in makeshift housing.

"It's been stressful," said Julia Ponder, executive director of the Raptor Center. She said it's not clear why there are so many injured eagles this year.

Lori Arent, left, and volunteer Janice Constable, examined a juvenile bald eagle at The Raptor Center in St. Paul before sending it out for a rehabilitative flight on Wednesday, Nov. 4, 2015.Tom Baker for MPR News

Quite a few are juvenile birds that were brought in following big summer storms. These eagles have been long-term patients, because their families can't be located and the young birds can't fend for themselves in the wild yet.

Ponder said caring for the young birds has cost a lot of money.

"Eagles are big," she said. "They take up a lot of space and they eat a lot of food." She estimated that food expenses alone were $5,000 to $10,000 above normal, and "that doesn't include staff time and the other medical resources."

Overall, Minnesota's eagle population appears to be doing well. The DNR estimates there are at least 1,000 nesting pairs in the state and 60 eagle nests just in the metro area.

More birds encountering urban hazards likely account for some of the increase in injuries. But that doesn't explain why there has been such a huge single-year jump in the number of eagles getting hurt.

Whatever the reason, the situation has put pressure on Raptor Center staff to rehabilitate its youngest eagles in time to join others that have migrated to open water in the Mississippi River near Wabasha. The eagles that are wintering there can show them how to hunt and fish.

Janice Constable, left, a volunteer with The Raptor Center, released a juvenile bald eagle at Como Regional Park in St. Paul, while volunteer Steve Masten controlled the line to which the eagle is attached. The two volunteers work with birds to help prepare them for flying once released back into the wild.Tom Baker for MPR News

As Masten released the young raptor for a test flight on its block-long tether, clinic manager Lori Arent scrutinized the flight position of the bird's wings, tail and feet.

It was only the eagle's second test flight since it arrived with a fractured breastbone in June. Arent was pleased with the bird's form, but she also wanted to test its endurance.

"A lot of times they do power out on their first flight, but then they get tired as time goes on," she said. "So their strength is really indicated by how many flights they can do before they need a rest. So we'll wait and see."

After the fourth test flight, Arndt noticed that the bird was cheating a bit. "It was just starting to get a little tired. It was doing more gliding," she said.

Standing in the middle of the windy park, Arent said she wanted to see all of the eagle's outings be high-powered, flapping flights without any gliding. The bird needed more work.

The young eagle has a little more than a month to get ready for its trip to Wabasha. Arent thinks it will be ready, but if not, the Raptor Center will take care of it as long as necessary.